1. Field of the Invention
The present invention relates to a broad band digital exchange applicable to a broad band ISDN (integrated services digital network), and more particularly to a broad band digital exchange with uses in ATM (asynchronous transfer mode).
2. Related Background Art
A prior art ATM exchange uses a Banyan network in which 2.times.2 unit switches are regularly arranged. The Banyan network is used as a switching element network for exchanging digital information. The connection state of each unit switch is switched in accordance with a destination address stored in each cell. Thus, the cells inputted to input channels of the Banyan network are outputted to predetermined output channels. If there is a conflict between the cells inputted to two input terminals of the unit switches because of the same destination addresses, one of the cells is preferentially outputted to the predetermined output terminal of the unit switch in accordance with the stored address. However, the other cell is outputted to an output terminal which is different from the intended output terminal specified by the destination address. As a result, the other cell reaches the output terminal which is different from the destination address.
Thus, in the switching element network of the prior art ATM exchange, the cell whose transmission path is not preferentially selected when the cells conflict is not outputted to the predetermined output terminal determined by the destination address stored in the cell. Hereinafter, the cell which is preferentially selected is called a won cell, and the cell which is not preferentially selected is called a lost cell. Once the cell loses, it becomes a totally invalid cell in the Banyan network. In each unit switch, the connection state of the switch is determined based on only one bit of the destination address. As a result, there is a possibility that a lost cell conflicts with a valid cell which has heretofore continuously won and wins to the valid cell in the unit switch of the next stage. As a result, the invalid cell in the Banyan network impedes the transmission path of the valid cell and it raises a cell discard rate in the ATM exchange.
In addition to the Banyan network, a Batcher Banyan network is also used as the switching element network in the ATM exchange. The Batcher Banyan network is provided with a sorting network in a preceding stage to the Banyan network.
In the Banyan network, the discard rate of the cell is raised when the cells conflict in the network, as described above, on the other hand, in the Batcher Banyan network, a probability of conflict of the cells is low but it requires 2.sup.N .times.N.times.(N+1)/2 2.times.2 unit switches in the Batcher switching network, where N is the number of input channels. As a result, the hardware requirements of the network increase and the system is of large size.